Removal of trace Cd super(2+ using continuous multistage ion foam fractionation. Part III - Effect of salt addition)

In this study, a multistage foam fractionation column with 5 bubble-cap trays was used to remove cadmium (Cd) at a low concentration (mg/L) with sodium dodecyl sulfate (SDS) used to generate foam. The effects of anions (NO sub(3) super(-), Cl super(-, and SO) sub(4) super(2-) and cations (Na) super(+), K super(+, Mg) super(2)+, and Ca super(2+) were investigated by addition of sodium salts and nitrate salts, respectively. The addition of each studied cation was found to increase the residual factor of Cd (Cd concentration in effluent/Cd concentration in feed) due to the effect of the competitive adsorption of added counterions with Cd) super(2)+ at the air-water interface of foam at a relatively high concentration of cation whereas, for all studied sodium salts, the residual factor of Cd decreased slightly with increasing counterion concentration at low concentrations. However, the added anions had a little effect. The effect of added cations and anions on the reduction of Cd removal are in the following order: Ca super(2+ Mg) super(2)+ [much greater-than]K super(+ Na) super(+) and SO sub(4) super(2)- [asymptotic to] Cl super(- NO) sub(3) super(-, respectively. The SDS recovery was found to increase significantly with increasing added salt concentration because the repulsion force among the head groups of SDS is greatly reduced by the co-adsorption of the counterions from added salt, leading to increasing SDS adsorption at the air-water interface of foam. The added salts of divalent cations were found to affect both Cd and SDS separation performance more than those of monovalent cations.)